dt-bindings: panel: rocktech,jh057n00900: Add myself as maintainer

I maintained the txt based bindings before yaml conversion and care
about the driver.

Signed-off-by: Guido Günther <agx@sigxcpu.org>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/9427a9c0e6aaf9fb375f7ecee6691ba491149d52.1597561897.git.agx@sigxcpu.org